Can I use the unit with DJ applications other than rekordbox dj and Serato DJ Pro?

The DDJ-XP2 is designed to be used as a general purpose MIDI controller.

If you want to use a DJ application other than rekordbox dj or Serato DJ Pro, you need to configure the MIDI mapping.

The DDJ-XP2 MIDI Message List is available [here].
For details on MIDI mapping, see the instruction manual of your DJ application.